디지털 홀로그래픽 기법을 이용한 식물성 플랑크톤의 유영특성에 관한 실험적 연구

Abstract

Microbial cells are easily found in geometrically confined environments. The swimming characteristics of microorganisms are influenced by the presence of boundary surfaces. In addition, the microbial world belongs to the low Reynolds number regime where viscosity has a predominant influence. The motility of microorganisms results from the drag-based thrust. Therefore, the biological and physical swimming performances are directly related with the viscosity of the surrounding fluid and the presence of a nearby solid wall. In this study, in-line digital holography technique was employed to analyze the swimming characteristics according to two prospective factors, viscosity and wall interaction. The viscosity effect of ambient working fluid on the motility and flagellar beating motion of dinoflagellate Prorocentrum minimum was experimentally investigated. P. minimum is a thecate desmokont dinoflagellate with two flagella that emerge from the flagellar pore at the anterior end of the cell. A digital holographic PTV(particle tracking velocimetry) technique was used to measure the swimming trajectories of hundreds of P. minimum cells and to extract several kinematic motion parameters. The viscosity of the surrounding medium was changed from 1.12 cP (natural sea water at 22 °C) to 11.31 cP by adding methylcellulose to increase fluid viscosity. In natural sea water, the swimming speeds of P. minimum cells (n = 77) in a helical motion is ranged from 5.4 μm/s to 138.4 μm/s with a mean speed of 51.3 μm/s ± 27.9 μm/s. The helix radius and pitch of the swimming trajectories are 3.08 ± 0.64 and 25.34 μm/s ± 7.96 μm/s, respectively. The longitudinal flagellum beats with a planar wave at a beating frequency of 87.10 Hz ± 10.96 Hz. On the other hand, the transverse flagellum beats with a helical wave at a beating frequency of 45.38 ± 13.61 Hz. As the viscosity of the ambient medium increases, the beating frequency of flagella decreased and consequently, the swimming speed of P. minimum is reducedWe also investigated 3D motile characteristics, especially in the near-wall region. This 3D imaging technique enables us to analyze the effects of cell and solid wall interaction on swimming behaviors such as swimming speed, trajectory, and attraction to the wall. Results show that swimming microorganisms tend to gather near the solid surface and to have a configuration parallel to the swimming direction to the wall. The cells also have faster swimming speed near the wall compared with that of unrestrained free-swimming ones.